1. Field of the Invention
The present invention relates to an apparatus for controlling a compressor used in an automobile air-conditioner.
2. Description of the Prior Art
Japanese Patent Laid-open Publication No. 60-22525 exemplifies an apparatus for controlling the displacement of a variable displacement compressor incorporated in an automobile air-conditioner and driven by an engine of the automobile. The apparatus includes means for setting a target cooling power of a cooler and means for measuring the actual cooling power of the cooler. The target cooling power is determined depending on various conditions such as thermal loads in the vehicle passenger compartment, a temperature set by an occupant, etc. The displacement of the variable displacement compressor is controlled by properly adjusting a displacement adjustment signal until the actual cooling power is in equal to the target cooling power.
According to the known control apparatus, the target cooling power varies with a change thermal load having an influence on the vehicle passenger compartment temperature or with a manual setting of the temperature by the occupant. A change in target cooling power directly varies the displacement of the compressor. Consequently, if the vehicle compartment setting temperature is greatly varied by manual settings of the occupant, the displacement of the compressor changes rapidly as a consequence of a sudden change of the target cooling power. Such a rapid change of the compressor displacement may increase the engine load which will affect the drivability of the automobile or provide an unpleasant feeling due to a sudden change in temperature of the discharged air.
A modified form of the compressor control apparatus is proposed by the present assignee, wherein the operation of the compressor continues even when the cooling power of the evaporator represented in terms of the temperature of the evaporator is below a freezing initiation temperature near 0.degree. C. This is achieved either by setting the target cooling power to an extremely low temperature such as -10.degree. C. to supply a maximum cooling power (cool-down control) when a quick cooling of the vehicle passenger compartment is needed, or by similarly setting the target cooling power to a low temperature to improve the demist performance characteristic (low temperature demist control) when the outside air temperature is low. However, a continuing operation under such a cool-down control and low temperature demist control is likely to cause a freezing of the evaporator which makes it impossible to return to the regular control mode.